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The convenience of wireless charging: Its just physics White paper
By Mark Estabrook, MediaTek
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Contents New hope emerges amidst standards war ................................................................................................... 2
Operator strategies for power management in LTE phones ........................................................................ 3
Why inductive charging may underwhelm consumers ................................................................................ 4
How tightly coupled technology is much like that of wired chargers .......................................................... 5
How tightly coupled technology constrains design ...................................................................................... 6
Summary on characteristics of tightly coupled inductive solutions ............................................................. 6
The convenience of loosely coupled highly resonant systems ..................................................................... 8
The theory behind highly resonant systems ................................................................................................. 9
Summary on characteristics of loosely coupled highly resonant solutions .................................................. 9
The efficiency of loosely coupled resonant systems .................................................................................. 10
Cost comparison of loosely coupled resonant and tightly coupled systems .............................................. 11
Communication between charger and receiver ......................................................................................... 12
Multimode solutions ................................................................................................................................... 13
Summary ..................................................................................................................................................... 14
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New hope emerges amidst standards war
A storm is brewing in the world of wireless charging. Over the past few years we have seen a number of
consumer products in the market from Powermat and members of the Wireless Power Consortium
(WPC) based on inductive technology, sometimes also referred to as tightly coupled technology.
Powermats efforts have led to the formation of another consortium called Power Matters Alliance
(PMA) ostensibly to compete with the large, well organized and well-funded WPC. Although the nod
goes to the WPC for the sheer number heavy weight companies supporting the specification, including
MediaTek, Nokia, Samsung, LG, Verizon, Philips, and NTTDocomo, the PMA has also lined up some heavy
hitting supporters including MediaTek, AT&T, Google, Proctor & Gamble, TI, NXP and Starbucks, with
many more joining recently. The PMA specification offers some interesting capabilities around
networking that can support active monitoring and metering of power to each device. This feature is
attractive to potential providers of charging services, like Starbucks. The PMA, with MediaTeks support,
is also aggressively pursuing an integrated dual mode approach to wireless charging, bringing the
resonance and inductive technologies under one unified and interoperable specification.
While industry pundits debate which of the inductive specifications will win the market acceptance
battle, other companies and groups, like MediaTek, A4WP, and PowerbyProxi, are talking about a newer
technology commonly referred to as resonance but more accurately described as highly resonant
loosely coupled technology. The providers of resonant solutions have yet to release a product, but are
showing compelling demonstrations at tradeshows and online.
None of these inductive or resonant specifications or demonstrations support, or will support products
that are directly compatible with the other. So its not just a replay of the BetaMax vs. VHS saga. Its
going to be an all-out street fight. Or is it? Will one technology or specification rise to the top?
While the inductive solutions have a strong head start in the marketplace, resonant solutions may offer
a significantly better user experience at lower cost. Geoffrey Moore in his highly acclaimed book
Crossing the Chasm describes the need for early products and technologies to hit several key
milestones in order to transition from the early adoption phase, where interest is high but volumes are
small, to the early majority phase, where a more discerning customer resides but volumes are high. To
cross the chasm to success, a product must be easy to use, widely available, and relatively low in cost.
The consumer is looking more for value and less for the wow factor. Inductive solutions today have
created a wow impact in the market and found early success with early adopters, but suffer from
severe limitations that are inherent in the physics of how inductive solutions work. Can inductive
systems provide ease of use and low cost? We dont think so for the majority of consumer devices
wanting to be wirelessly charged (e.g. phones, BT headsets, tablets, phablets, cameras, smartwatches,
notebooks, etc.). This paper tries to give some background to the underlying physics behind inductive
and resonant technologies that lead directly to their ability, or not, to cross the chasm of market
success.
But before we compare and contrast the inductive and resonant technologies, we should first answer
the simple question of why wireless charging makes sense at all. Compared to wired charging, all
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wireless charging technologies have lower efficiency, higher cost, and may not be able to charge at the
same rates of power. So there needs to be a compelling reason to want wireless power beyond just the
wow factor.
Operator strategies for power management in LTE phones
Operators have been working closely with the many of the players in the mobile phone eco-system to
develop strategies to lessen power consumption, but the reductions are not enough. The operators are
looking to the promise of convenient and frequent charging with wireless power as one more key
strategy for keeping the phone battery full. Note that Verizon, AT&T, NTTDocomo, Softbank and KDDI
are all early adopters of both LTE handsets and wireless power.
You might think that consumers will not accept hourly charging in place of daily charging, but this is
precisely the hope of wireless charging. Lets face it, plugging a USB cable into your phone is not that
hard. If you only do it once a day, is it really worth the US$50 to $75 per phone you will pay for the
convenience of placing the phone on a charging pad rather than plugging in the USB cable? Probably
not. So what the operators are looking for is a way for customers to easily charge their phones without
thinking too much about it. Charge it at home, in the car or subway train, in the coffee shop and in your
office. Place the phone in the right spot anywhere and let it charge. But you need a couple of things for
this to be an experience embraced by the consumer: Interoperability between chargers and receivers
(phones, tablets, etc.), and a compelling user experience that is far better than plugging in a USB cable.
A good user experience includes:
Ease of use: Placing the phone on or near a charging pad or surface without worrying about
alignment or movement of the device from bumps or vibration.
Reasonable power: The device should charge at a rate similar to wired charging or better.
Low cost: The benchmark is the cost of a USB cable, which wireless chargers will never meet.
Consumers, however, will always weigh the cost of a cable against the convenience of wireless
charging.
Interoperability: Single device: Any phone should charge on any charger.
Multiple devices: A single charger should charge all your devices concurrently (at least phones,
headsets, and tablets).
Safety: There can be no safety risk in terms of heating or emissions.
Environmentally friendly: Several markets and cultures demand environmentally friendly
solutions that do not consume excessive power.
Perhaps a truly disruptive technology is required to provide these compelling user experiences.
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Figure 2: Amperes Law
Why inductive charging may underwhelm consumers
Powermat launched its first products for wireless charging in 2009. Powermat technology is the basis of
the specifications being developed by the Power Matters Alliance (PMA). The Wireless Power
Consortium (WPC) members launched products starting in 2011 in Japan. Both products have seen early
successes with market adoption that is due, at least in part, to the wow factor of wireless charging. An
important question remains as to whether the technology is
disruptive and compelling enough to cross the chasm
between early adopters and early mass market adoption.
In attempting to answer this question lets first look at how
inductive charging systems work.
The electrical model of a transformer can be used to
describe a system. Transformers have primary and
secondary windings and an iron core that couples the
magnetic field of the primary into the secondary winding.
The current flowing through the primary coil creates a magnetic field according to Amperes law:
dB = change in magnetic flux , I is the current in the wire, dl is the length of the wire, sin is the angle of
a point from the wire where the magnetic field is measured, r is the distance to that point, and k is a
constant (see Figure 2 )1.
The primary coil is coupled into the secondary coil via the induced
magnetic field. The purpose of the iron core is to help collect the
magnetic field around the primary winding and present it to the
secondary coil. The induced magnetic field in the secondary coil
creates current flow (wireless power transfer) in the secondary
coil.
In a tightly coupled wireless charger the principle is the same but
with the iron core removed and the use of planar coils rather than windings. Without the iron core the
magnetic field must travel across air rather than iron. Air has much lower permeability (ability to pass
magnetic fields) than iron by a factor of about 7,0002. Therefore the amount of magnetic flux and
resulting power coupled into the secondary coil across air is much lower than with iron. In order to get
power transfer with any reasonable degree of efficiency (typically about 70 percent DC in to DC out in
todays wireless power systems) the primary and secondary coils must be in very close proximity and in
1 McGraw-Hill Science & Technology Encyclopedia: Ampre's law.
Figure 1: Transformer model
Source: http://nuclearpowertraining.tpub.com/h1011v4/css/h1011v4_48.htm
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concentric alignment. This is so that the secondary coil couples to the largest and strongest part of the
primary coils magnetic field. In more specific technical terms, any magnetic flux from the primary that
does not couple into the secondary is represented as leakage inductance. Leakage inductance causes
energy to be wasted because it presents an impedance to the source coil driver but does not induce
voltage in the secondary winding. The increased impedance in the source coil driver causes I2R losses as
the current is increased to maintain charging at target rates in the receiver.
In normal use the leakage inductance would be minimal. But if you try to move the primary and
secondary coils away from each other the leakage inductance
becomes much greater which we measure as a decrease in
coil to coil efficiency. At lower coil to coil efficiency the
charger must develop and attempt to send more power
leading to higher charger losses for the same delivered
power.
Thus the primary and secondary coils must be arranged in
such a way as to always remain tightly coupled. Typical
inductive coils can be seen in Figure 3. Note that the coils are
very similar in shape and size. These particular coils are used in a 2.5W smartphone charger.
Primary (charger) and secondary (receiver) coils in tightly coupled wireless power systems are:
Concentrically aligned
Approximately the same size
Used in very close proximity (very little distance between the coils)
How tightly coupled technology is much like that of wired chargers
Looking at these characteristics you can see that tightly coupled systems couple power from a single
primary coil to a single secondary coil. This means that tightly coupled
chargers are 1:1 charging systems. They charge only one device at a time.
Notably, USB charging cables are also 1:1 chargers. You need one charger
for every device. If you want to charge more than one device concurrently
you will have to use multiple chargers. To charge two phones concurrently
youll need:
Two AC adapters for wired charging
Two AC adapters and pads for wireless charging3
3 There are some chargers that have multiple charging pads for similar devices and just one AC adapter, but you cannot get
away from needing at least one active charging coil for every device.
Figure 3: Example Qi primary coil (right) and receive coil (left)
Figure 4: Wired vs. Qi wireless charging for two phones
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Based on this fact alone inductive wireless charging does not seem to be a potential disruptive
technology that can provide a low cost, convenient charging experience.
How tightly coupled technology constrains design
Finally, tightly coupled charging systems must have the primary and secondary coils placed in very close
concentric alignment in a flat plane and remain in very close proximity with each other to charge
effectively. The mechanical design of some products dont always allow for easy alignment. Phones and
tablets require that the flat coil be placed on the back cover of the device. In most cases that works very
well. But other devices like smartwatches and Bluetooth (BT) headsets dont lend themselves to this
physical limitation. As weve seen, BT headsets need to use charging cradles that are large enough to
accommodate the coils size. But the secondary coil also has to sit flat on the charging pad rather than
just lay on the charging pad in no specific manner. This means that in addition to purchasing a charging
pad with an AC adapter, the consumer must also purchase a charging cradle for the BT headset in order
to use wireless charging. With most BT headsets you simply plug a mini-USB charger into the headset
itself. Which would you prefer? Keep in mind that with tightly coupled inductive 1:1 charging systems
you cannot charge both your BT headset and your phone at the same time4.
Summary on characteristics of tightly coupled inductive solutions
If there is only the requirement to wirelessly charge a single phone then a tightly coupled charger should
work quite well. WPC and PMA products have had the benefit of a few years of engineering
development that have led to well optimized small footprint, low cost, and high efficiency designs and
even some freedom from critical concentric alignment if you use a multi-coil charger design5 (albeit at
higher cost). But to cross the chasm from early adopter to early mass market wireless charging solutions
the charging experience should be easy and convenient to the point of being almost thoughtless. This
means being able to charge multiple devices at a time, charge different sized devices at optimal power
(Bluetooth headsets, phone, camera, tablet, etc.), and to not restrict mechanical designs optimized for
consumer convenience and aesthetic value.
4 Some inductive chargers include multiple coils and electronics to support multiple device charging (up to two as per whats
on the market, and five as per demonstrations). They are still 1:1 charging systems, but simply combine two or more separate chargers into one package. 5 The charger is able to identify which charger coil is in good alignment and energizes that particular coil to enable 1:1
charging.
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Tightly coupled inductive wireless power solutions have the following key characteristics:
Inability to charge more than one device at a time
o Result: Consumers must purchase multiple chargers to charge devices concurrently.
A need for coils on the charger side and the receiver side to be of similar size and shape
o Result: Secondary receiver coils cannot be matched to the receiver device for
optimal power delivery. With charges delivering moderate power, low power
devices cannot be charged and high power devices can only be charged at the
moderate rates.
o Result: Secondary receiver coils cannot be matched to the device for best
mechanical fit (size and shape such as circle, square, rectangle, oval, etc.).
A need for charger and receiver coils to be closely aligned
o Result: Only specific and tightly constrained mechanical designs can be considered.
Some devices like Bluetooth headsets cannot practically be charged at all without
the use of a separate charging cradle that houses the receiver coil.
The simple laws of physics lead us to certain undeniable facts. Tightly coupled systems have severe
limitations in optimally charging more than a single style of consumer device under a single standard,
and can only charge one device at a time. No amount of marketing promotion and market hype can
change this fact. And as John Adams famously said: Facts are stubborn things. So what can be done?
Will the early success of wireless charging fail to gain momentum and cross the chasm to the mass
market?
No. A basic principle in physics comes to the rescue: Resonance. If we could design a system where the
secondary coil needs only intersect with a limited number of field lines from the primary coil, then the
secondary coil would not have to be aligned precisely with the primary coil, or be the same size. The
secondary coil can perhaps even be moved some distance away from the primary coil. And if the
secondary coil can move away from the primary coil, then there is the possibility of more than one
secondary coil to intersect with the primary coil field lines. This sort of system would be called loosely
coupled. The problem with loosely coupled systems is that the efficiency of the power transfer will be
very low since the magnetic flux lines intersecting the secondary coil are fewer and typically weaker
than in a tightly coupled system. But there is a principle in physics called resonance that can greatly
improve the efficiency of power transfer and can make loosely coupled wireless charging practical.
Highly resonant loosely coupled systems can make a truly low cost disruptive charging solution that can
cross the chasm to mass market adoption.
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The convenience of loosely coupled highly resonant systems
In a loosely coupled system the secondary coil may be coupled to fewer of the magnetic field lines and
have a larger distance from the primary coil as compared with inductive coupling. This provides a
significant degree of highly desirable spatial freedom between the primary and secondary coils. Yet
based on the model for a tightly coupled system, this arrangement should lead to very low efficiency.
And it does, unless the coils are highly resonant. With highly resonant coupling of the magnetic field
between the primary and secondary coils, efficient power transfer can be achieved between the coils.
What is this magical thing called resonance? Its a principle in physics thats been known
and studied for many years. The classic example of resonance is the opera singer that sings
and holds a specific musical note. Across the room are glasses filled to different levels to
create receivers that will resonate at a particular frequency based on their physical
characteristics set by the level of liquid in each glass. The
glass that is resonant at the same frequency of the Opera
singers voice shatters due to the absorption of energy
from the Opera singers voice. Notice that in this
example those glasses that did not resonate with the
singers voice did not absorb the same energy and did
not shatter. In nature many objects have a natural
resonant frequency based on their physical
characteristics. In the electronics domain we can create a
resonance with the proper selection of resistance,
inductance and capacitance. Most electronics designers
have unintentionally created a resonant circuit at some
point in their career with unintended and disappointing
results.
Nikola Tesla demonstrated that the principle of resonance could be used for transferring power over the
air back in the early 1900s6. Power by Proxi has been applying these principles to build loosely coupled
systems for industrial markets with some success over the past few years. Solutions aimed at phones are
being developed and demonstrated by companies, including MediaTek, Intel, Qualcomm and Power by
Proxi, though no commercial products that have yet to be released.
In electrical systems, resonance is achieved by the appropriate design of a Resistance-Inductance-
Capacitance (RLC) circuit. Resonance occurs at a particular frequency determined by the RLC values. The
higher the Q (quality factor) of the circuit the higher the efficiency of energy transfer. The inductive
reactance (L) and capacitive reactance (-1/C) will be of equal magnitude at resonance where the
Quality factor (Q) is then largely determined by the resistance in the circuit.
6 http://en.wikipedia.org/wiki/Nikola_Tesla#Later_years_.281918-1943.29
Figure 5: Nikola Tesla pictured with his tower used in loosely coupled resonant magnetic power experiments
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=2f where f is the frequency of resonance
We find it easier to achieve high Q at higher frequency and this is why you see so many developers of
resonant systems operating at 6.78MHz. The WPC and PMA do not rely on high Q circuits and operate at
lower frequencies (100s of kHz) where circuit design is simpler.
The theory behind highly resonant systems
Resonant systems are very similar to inductive systems in that there is a primary coil and secondary coil.
The difference is that with loosely coupled
resonant systems the secondary coil is not
dependent on being in close proximity to a large
percentage of the B field coming from the
primary coil so long as high Q coils are used. In
other words, effective power transfer is not
strictly dependent upon alignment, size, shape or
positioning of the secondary coil to the primary
coil. In addition, and perhaps most importantly,
multiple secondary coils can also be used to
capture power since each coil can share the
overall coupling with the primary coil and still
have efficient power transfer according to the equation presented in the previous paragraph which
graphically illustrates this principle. As shown, the charging of multiple devices, or 1:many charging, is
inherently supported. People watching demonstrations often describe the loosely coupled systems as
magic but, of course, its just cleverly applied physics.
Summary on characteristics of loosely coupled highly resonant solutions
The physics of highly resonant, loosely coupled systems affords some unique properties that may enable
the industry to deliver truly disruptive and compelling solutions to consumers. Resonant systems can
ensure we can keep our many different portable electronic devices charged and ready for use at all
times. Resonance may be the key that drives early market adopters of inductive wireless charging to
cross the chasm to the mass market and associated high volume purchases.
Figure 6: Inductive vs. resonant wireless power transfer
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Loosely coupled resonant wireless power solutions have the following key characteristics:
Primary and secondary coils of different sizes
o Result: Secondary receiver coils can be matched to the receiver device for optimal
power.
o Result: Secondary receiver coils can be matched to the device for mechanical fit (size
and shape such as circle, square, rectangle, oval, etc.).
Multiple devices can be charged on a single coil charger that enables 1:many charging
o Result: One charger operating with a single coil under one standard can support
charging of Bluetooth (BT) headsets, phones, cameras, tablets, etc.
Devices can be placed on the charger pad in almost any orientation
o Result: Almost any mechanical designs can be accommodated. For example, with BT
headsets, you can simply place the headset onto the charging pad, rather than placing
into a charging cradle that aligns to the planer charging surface.
The contrast between loosely and tightly inductive solutions is pretty dramatic. But the comparisons
thus far are not complete. We need to also look at efficiency and cost.
The efficiency of loosely coupled resonant systems
Efficiency is an important consideration for wireless power. One can always deliver the target power
almost regardless of the efficiency, but at what cost and size? The greater the efficiency the smaller the
size and cost of the charger for the same delivered power. In the case of a smartphone the wired power
has around 97 percent efficiency as measured
from the wall socket to the 5V output to the
battery. Wireless power efficiency will be less
efficient. How much less depends on many
factors including how large of a distance we
want to charge over. Many people have the
pre-conceived notion that loosely coupled
resonant systems are lower efficiency than
tightly coupled inductive systems. This is not
the case. When comparing the two
approaches on an equivalent basis resonance
will have a slightly higher coil to coil efficiency.
As the distance between the primary and
secondary coils increases the efficiency of
either system will decrease, but the decrease
is much slower with loosely coupled resonant
Figure 7: Efficiency comparison of simulated tightly coupled inductive and loosely coupled resonant coil to coil systems (see footnote for absolute coil sizes)
0
10
20
30
40
50
60
70
80
90
100
0 20 40 60 80 100
Effi
cie
ncy
(%
)
Z separation (mm)
Inductive (1:1)
Resonance (1:1)
Resonance (12:1) Corner
Resonance (12:1) Center
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systems thus providing a significant advantage over inductive systems. This can easily be seen in
Figure77 where the coils sizes are the same (1:1). However the really big advantage in efficiency for
resonant systems comes when you have primary and secondary coils of different sizes such that you can
support multiple receivers from a single primary coil. To charge three phones concurrently you might
have a single primary coil in the charger that is 12 times larger than each of the three secondary receiver
coils in the phones. The efficiency of a loosely coupled resonant system in this example is also shown in
Figure 7 (12:1). Tightly coupled systems cannot operate in these conditions and therefore no data is
presented. We can see from the curves presented that the optimal coil efficiency is always obtained
when the primary and secondary coils are of the same size and positioned very closely together.
Around the coils you have the usual electronics that include a regulator, driver and matching network in
the charger and a matching network, rectifier and regulator on the receiver side for both the resonant
and inductive systems. The resonant system requires a buck regulator in the receiver section where
some inductive solutions employ an LDO. The LDO has a slight advantage in efficiency as compared to
the buck regulator. The inductive systems can use LDOs because the input voltages are well controlled
since the receiver is always and necessarily in a 1:1 charging condition with tight coupling between the
two coils. Resonant systems have a wide range of voltages presented to the regulator because of the
varying coupling levels associated with spatial freedom and number of potential receivers in the field.
Cost comparison of loosely coupled resonant and tightly coupled
systems
As already described, loosely and tightly coupled systems are quite similar in design. They will also be
quite similar in cost and size. Both systems include the following blocks on the receiver side:
Coil (PCB or Lizt)
Ferrite
Rectifier (often synchronous)
Regulator
Digital Processing
As mentioned the regulator in the resonant system is likely to be a buck regulator which requires the
use of an inductor sized to the power requirements of the circuit. This inductor represents a slight
increase in cost and size of implementation compared to an LDO based regulator.
7 Based on simulation data (in mms) using 1:1 coils that are 35x35:35x35 and 12:1 coils that are 171x130:54x36.
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Communication between charger and receiver
For resonant systems the communication architecture may become a strong differentiator between
competing systems. It is widely accepted that a wireless power system requires some communication
between chargers and receivers to ensure a good user experience. Communications can be used to
adjust for optimum source power level, distinguish between valid devices and foreign objects, and
indicate fault conditions, to give a few examples. One-way communication from receiver to charger can
offer most of these benefits, but two-way communications makes for more efficient use of the channel.
With todays tightly coupled charging systems these communications are done in-band. This means
they use the same magnetic field required to transfer power as the carrier for communications (usually
small changes in amplitude), much like RFID tags. The alternative would be to use a separate short range
radio technology to communicate out-of-band. The benefit of the in-band approach is that there is
minimal cost and space required in the system to support communication. In addition, the system is
inherently reliable, stable and free from external influence from other chargers. Communication only
occurs when power is transferred from the charger to the receiver. In-band has another benefit for
consumer devices in that dead battery conditions are inherently supported in the architecture because
the system is self-powered.
Some suppliers of resonant systems will undoubtedly try to bring in-band architectures to resonance.
However, implementing in-band solutions within loosely coupled systems is a difficult engineering
problem due to the less predictable coupling levels between chargers and receivers. Any solution will
require clever circuit design to create a well behaved system and advanced communication algorithms
to pull a very small signal from a lot of system noise. The A4WP (loosely coupled solution) has publicly
announced their adoption of an out-of-band communications architecture based on Bluetooth Low
Energy (BTLE). This approach has the benefit of being an easier engineering problem to solve compared
to in-band implementation, but unfortunately includes the higher cost and board area associated with
BTLE transceivers at both the charger and receiver. You could save the cost of the BTLE in the receiver if
you used the BT radio already present in the device (e.g. phone or tablet), but this is more complicated
for the device design for a few reasons:
1. A data path must be established between the wireless power receiver and the Bluetooth (BT)
radio on the phone (requiring one more set of interconnects and software overhead tasks).
2. The phone OS must take responsibility for communications associated with power control.
3. Dead batteries are not supported unless a dedicated power line is brought in from the wireless
power receiver to the on-board BT solution. Power management systems today are not
designed for this approach.
Compare these complexities with implementation of an in-band solution where the outputs of the
wireless power receiver are V+ and Ground thats it.
At a system level there are also concerns with chargers using out-of-band architectures interfering with
each other. Chargers using BT have been shown to indiscriminately communicate with receivers
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associated with other chargers. This situation can lead to both operation and safety concerns. Chargers
that use in-band architectures only communicate with receivers that they are actively associated with
for charging.
One of the main challenges associated with adopting in-band architectures for loosely coupled systems
has to do with the expected variation in coupling between the charger and the receiver. Inductive and
resonant systems have comparable coupling and efficiency levels when the receiver is placed in the
optimal charging position. However, resonant systems allow movement of the receiver away from the
charger while still charging, albeit with lower coupling and efficiency as shown in Figure 8. Variations in
coupling can be managed with good communications systems design. However with decreasing coupling
levels the communications, signals will become increasingly small and eventually get lost in the system
noise. Therefore loosely coupled systems using in-band communications will need to define a point of
minimal coupling beyond which the communications and thus charging will cease. This point should in
most applications, such as phones and tablets, be well beyond that of minimally acceptable power
transfer efficiency. Applications where very low power efficiency is less of a concern, such as AA
batteries, mice, or keyboards, might be served better with no communications (open looped power
transfer) or out-of-band communications.
In summary, loosely coupled systems that support in-band communications will have a clear competitive
advantage over systems using out-of-band architectures if the technical problems can be adequately
addressed. These advantages include:
Lower cost
Smaller PCB area (important in phones and wearable devices)
Higher reliability (power and communications are integrated)
Lower complexity (power and communications are all self-contained)
Multimode solutions
The best situation for consumers in markets like that for wireless power is to have a single global
standard specification so that all receiver and chargers are interoperable. But new industries dont often
develop that way and wireless power is no exception. As previously mentioned, Powermat came to
market first. Various suppliers launched products under the WPC specification some time later. But
products based on each specification do not work with each other. If you buy a Powermat enabled
phone you cannot charge it on a Qi certified charger. However industry suppliers like TI and IDT have
developed dual-mode solutions where in this example a phone using a TI dual-mode receiver can charge
from either a Qi or Powermat certified charger. With TIs solution there is no significant cost penalty for
this feature in terms of bill of materials and size. So dual-mode solutions can be a great way to increase
consumer convenience at low cost. Since the wireless charging market is not yet rallying around a single
specification or standard we believe that the trend towards availability of dual-mode solutions is
positive for the market.
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When loosely coupled systems are introduced to the market in the coming months they will be
incompatible with existing tightly coupled products. Can the principle of dual-mode be extended to
multi-mode where a phone or tablet can charge from either a loosely or tightly coupled charger?
Certainly the answer is technically yes, but at what cost? Can the multi-mode function be realized
without significantly increasing the cost of the coil structure or the bill of materials? We think yes. Any
such system will have to make sure that both the tightly and loosely coupled modes will provide for:
- Quick charging start times irrespective of charger specifications
- Low cost
- Small size
- Consistent, efficient power transfer
The complications of developing such a multi-mode system are beyond the scope of this article, but
suppliers who can meet these targets will likely find strong market acceptance for their solution. In fact,
low-cost, multi-mode solutions that protect the investments made in todays inductive charger
infrastructure while offering the advantages of resonant charging may be critical in the market transition
from early adoption (mostly inductive based) to early majority (mostly resonance based) phase.
Summary
Todays wireless charging systems have shown success in the market place with early adopters. These
early adopters are consumers who want the latest and greatest technology, especially technology with a
wow factor, almost regardless of price and convenience. The road is littered with failed products that
do not make the jump from the early to mass market adoption phase. In following Geoffrey Moores
advice on how successful products cross the chasm to mass market success we look for truly innovative
and disruptive technology that drive costs down and convenience up disruptive technology that
changes the way we charge consumer devices, such that it is seamless and available at costs palatable
for high volume consumer markets.
Todays tightly coupled inductive wireless charging solutions are underwhelming to people more
interested in the product (the mass market) than the technology (early adopters). Qi and PMA products
users must still think about and plan for the charging of their devices, and do so at significant cost, given
the need to purchase individual chargers for each device for concurrent charging and support of
different classes of device (e.g. Bluetooth headsets, phones, tablets). Although inductive systems will
undoubtedly continue to improve in performance and function, the physics that underlie the technology
may prevent these products from ever improving enough to reach the high volume mass market
adoption stage despite the significant marketing dollars being spent.
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But if the techniques of loosely coupled highly resonant systems are applied to wireless charging we can
see a path to a truly disruptive charging experience for consumers. The resonant technology will allow
for a low cost, low complexity system with a single coil on the charger and receiver to:
Charge multiple devices concurrently
Charge different devices with vastly different power needs
Charge different devices with vastly different form factors without coil-to-coil alignment
Charge devices with easy, imprecise placement of the device on or near the charger
Charge devices across distances and through furniture or even walls
The two major inductive consortiums, the WPC and the PMA, have recently recognized the importance
of resonance. Each consortium is welcoming new members and new roles presumably to add resonance
capability alongside their inductive technologies. MediaTek has joined the PMA as co-vice chairman of
their new resonance technology working group. PowerbyProxi has joined the WPC and may lend their
approach on resonance to the organization. MediaTek is already a member of the WPC and can be
expected to also support the development of resonance within the WPC. Intel has recently thrown its
weight into A4WP as a board member. Qualcomm has joined both the WPC and PMA groups. All major
wireless power players are investing in resonant technology for the future.
Resonant charging systems are in their infancy and may have some early growing pains. But the benefits
afforded by the underlying physics of this technology will be help bring the early success of inductive
wireless charging across the chasm to mass market adoption.